The value of the prescale of the PCA9552 may not be accurate

[yong_huang]

Hi 

     We develop the NXP pca9552 which controls the leds on our board under linux enviroment.

     We want the leds blink on pwm0 1HZ frequency ,so set the pwm0 prescale register value to 43,according to the datasheet 6.3.3,   https://www.nxp.com/docs/en/data-sheet/PCA9551.pdf  ,the blink rate should be ((PSC0 + 1) / 44 = (43+1) /44 =1 hz

     But when we measure the waveform of the leds, the blink rate is 1.2 second, not 1 second.

     After looking the leds-pca955x.c source file in linux driver ,it notices "  Write to frequency prescaler register, used to program the period of the PWM output. period = (PSCx + 1) / 38 "   

     Then we set the pwm0 prescale0 to 36, the blink frequency is 1 second ,is more accurate then set it to 43 before 

    From the the pca9551 datasheet ,it notice the blink rate  is (PSCx + 1) / 38  PCA9551 | NXP 

     We are confused if the PCA9552 pwm prescale register blink rate should also be  (PSCx + 1) / 38 ,  not be  (PSCx + 1) / 44 ? The datasheet may be not right ,according to our measure.

     Look forwarding your help

[TomasVaverka]

Hi,

The PCA9552 is a very old device and it is hard to verify this BLINK0 period. I do not have an evaluation board for the PCA9552, but I will order PCA9952 samples to be soldered on the breakout board to perform some measurements. Then I will get back to you with results.

Thank you in advance for your patience.

Best regards,

Tomas

[yong_huang]

Hi Tomas,

      Thank you. Look forward to the result.

[TomasVaverka]

Hi Huang,

I have not received the samples yet. In the meantime, could you please let me know how many parts have you tested so far? Do all the measured parts have 1.2 seconds blink rate (while setting is 1 second)?

As there is no specification on the Osc accuracy, I am afraid that if you require accurate blink rate you will need to calibrate each board PCA9552 Osc and store in EEPROM in uC to adjust to get accurate output.

Best regards,

Tomas

[yong_huang]

Hi Tomas,

     We test eight pieces of our board with prescale 44, the led blink rate are from 1.17s~1.23s.

     We test these boards with prescale 38 again,the led blink rate are from 1s~1.04s